So, Standard Template Constructs. The new trilogy of Warhammer 40K books are deliberately vague about what STCs are, exactly, which is certainly cool for purposes of piquing peoples' interests and makes sense story-wise since pretty much no one has actually seen one. But I am curious: is there some canon description of what STCs, well, look like? Are they merely very-detailed schematics? Huge machines that can assemble one type of thing using nanotechnology? Are they AIs? "GECK"-style boxes, a la "Fallout"? Bueller? (As a GM, I obviously have no problem simply coming up with my own idea of what an STC is and using that. But I'm interested to know if there is an "official" version of STCs, or what others have used in the past.)
My understanding is that Standard Template Constructs are items produced by autofactories according to a standardized set of simple, extremely robust designs (think of the AK-47) that also readily accept modification. The original point of them in-setting was increasing the survivability of colonies by giving them a library of reliable, readily manufacturable tools, vehicles and structures. The point of them on the meta level is, largely, justifying 1) having a relatively small set of basic designs, despite the enormous size of the Empire, with numerous variations on each, and 2) model and terrain conversions using components like Solo cups and poster tubes.
They are the sum total of Mankind's capabilities from the Dark Age of Technology. The STC was created in response to the wave of colonization that Mankind engaged in. Essentially, it was a system whereby the colonists could select their desired output (a Rhino, or a Power Plant, or whatever) and the system would automatically adjust its design to match the available materials at hand. In this way, humanity could colonize any planet they cared to, and their STC library would adapt itself to the raw materials and auto-construct everything they needed. However, as the Dark Age of Technology came to its horrifying conclusion, all of the known complete STC sets were lost (to the public eye; theories abound as to secret factions with some in their possession). What has been recovered, over time, are individual STC templates; they form the very backbone of the "modern" Imperial technology level. The recovery of a complete, intact, operational STC library would be an absolutely revolutionary event; especially when you realize that the very first likely output of it would be... ADDITIONAL complete, intact, and operational STC libraries. The fact that mankind holds, grimly, and with great expense of lives and treasure, to its holdings in the current storyline speaks to their determination and refusal to surrender; were they to find themselves re-equipped with limitless supplies of their forefather's weaponry and industry, the pitiful alien races who pretend to claim the right to exist in the face of the Emperor's scorn would find their judgement day upon them, and his holy cleansing wrath would scour them from the very face of existence.
Sorry, got carried away. You can find more information on the Lexicanum, if you like.
Standard Template Constructs, per the various fluff, are those items produced by autofactories from "standard templates". These range from ammo to Land Raiders.
The Contstructors themselves are only vaguely described; we know they can produce land raiders, or at least their parts for assembly. This means they need to be able to produce large items. The one description I've seen of one was as a complex of buildings. We know they are computerized.
An STC is referred to very few times in the W40K literature, but the most prevalent is that a true STC is an AI construct that can literally produce anything from the resources at hand.
It must be emphasized that Horus the Warmaster persuaded much of the Mechanicus to join his heresy with the promise of 2 INTACT STCs. To that end, he made war upon the Auretian Technocracy. Regulus says that the STCs were what drove man to the stars and that over time, the STC would produce weapons that would be able to counter their own weaponry.
This implies that they had artificial intelligence. The Auretian Technocracy had Thunderhawks, Astares armor, bolters and even Stormbirds. I would conjecture that the STCs built them during the conflict with the Empire, since the development of Astares armor took place after the unification wars. Therefore, they were more than just constructors. Personally, I believe that the short story, Ancient History, contains the key to what STCs really were. Kron relates the story of man's journey to the stars. He is at least 4000 years old. He describes how Man created the Stone Men to go to the stars and they in turn, created the men of Steel to be their arms and hands. It is the Stone Men who planted man's seed on many worlds. What if the Stone men and men of Steel were the original STCs. Standard Template ONE = Stone. Men of Steel = (S)tandard (T)emplate (eel). At the end of the story, the narrator, Nathan, falls unconscious after having his arm ravaged by a chainsword and his eye has been taken. When he wakes up 5 hours later, Kron says that he has fixed him up with what was at hand. He is in no pain, his arm is bandaged and he now sports a cybernetic eye and sees perfectly. Perhaps only an STC would have both the medical know-how to fix both his arm and create a cybernetic eye from what was at hand.
The STC are repositories of all of mankind's technical knowledge, created at the peak of that knowledge during the Dark Age of Technology. It's unclear if they merely store this information or if they also possess construction capabilities. Even if they did have limited construction capabilities, it's unlikely that they contain the capability to build everything they know how to (for example, Dyson Spheres and other megastructures).
They almost certainly do not contain AIs (though they probably have expert systems loaded into them), but they probably contain instructions on how to build AIs, along with all sorts of other forbidden technology. The AdMech are extremely wary of AI, and it's unlikely the STCs would have assumed the prominence they have in the religion if they were linked with the Iron Men.
Personally, I favour the idea that they are small and without ornamentation, but that's a personal aesthetic thing that I use in my games to distinguish DAoT technology from Imperial tech, not anything from canon.
I believe the STC is vague in the 40K canon as a whole. There's a Wikia link with some information from the novels. But it sounds to me like each ST Constructor makes a single type of technology. It's sort of an automated factory.
Like a lot of lore in 40K, the STC is lost mostly into mythology. The majority of imperial subjects wouldn't know what one is and probably maybe only the highest ranks of the Astartes, Mechanicum, and Inquisition(and of course the Emperor) would be able to recognize one on site.
Of course anything it can do has been passed down by oral tradition for 20,000 years. Pretty much all "history" in 40K is unreliable. It's even possible that they never existed, and they where just a lie created by the Emperor to gain the support of the Mechanicum--Though, that's entirely based on fridge logic.
Thematically it's more of a holy grail for the setting than an actual functioning technology. It's a connection to the advanced society that was destroyed in the Age of Strife, and was promised to be returned and made better by the Emperor's crusade.
This is a great story hook for any 40K RPG campaign. Because you can use anything for the maguffin, and it should always turn out to be some type of red herring. Being stuck in an unending quest is pretty much SOP for the setting.